It has been known that a film obtained by adding cerium (Ce) to indium oxide has a high refractive index and is a useful material in designing an optical film.
For example, Patent Document 1 discloses a sputtering target containing 10 to 40% by mass cerium oxide and the balance of indium oxide, that is, an In—Ce—O-based sputtering target having an atomic ratio of Ce relative to In and Ce in total, Ce/(In+Ce), set in a range of 0.082 to 0.35. The use of such a sputtering target makes it possible to obtain a film having a refractive index of 2.0 or more (the film is for protecting an optical disc).
Meanwhile, the sputtering target is prepared from an In—Ce—O-based oxide sintered body obtained by sintering a powder mixture of an indium oxide raw material powder and a cerium oxide raw material powder having been molded into a compact by cold isostatic pressing. However, if the sputtering target prepared in this manner is used for a long period, this brings about problems that nodules are likely to be formed on the target surface, and that abnormal discharge occurs.
Moreover, Patent Document 2 describes an In—Ce—C-based sputtering target having an atomic ratio of Ce relative to In and Ce in total, Ce/(In+Ce), set in a range of 0.005 to 0.15, in which cerium oxide particles dispersed in indium oxide have a diameter of 5 μm or less, so that the aforementioned abnormal discharge can be prevented.
Nevertheless, in order to obtain an optical film having a high refractive index (the refractive index is, for example, 2.1 or more), Ce needs to be added in a large amount. It is stated that, in increasing the amount of Ce added, if Ce/(In+Ce) exceeds the upper limit of 0.15, the particle diameter of the cerium oxide particles exceeds 5 μm, causing abnormal discharge (see paragraph 0028 of Patent Document 2). Hence, increasing the amount of Ce added results in such a problem that it is difficult to obtain a favorable sputtering target capable of preventing abnormal discharge.